Method and apparatus for NMR imaging

ABSTRACT

The subject invention pertains to a method and apparatus for Nuclear Magnetic Resonance (NMR) imaging. The subject method and apparatus are advantageous with respect to the use of RF coils for receiving signals in NMR scanners. The subject invention can utilize multiple coils to, for example, improve the signal to noise, increase the coverage area, and/or reduce the acquisition time. The use of multiple smaller surface or volume coils to receive NMR signals from the sample can increase the signal to noise ratio compared to a larger coil that has the same field of view and coverage area.

BACKGROUND OF THE INVENTION

The subject invention relates to the field of Nuclear Magnetic Resonance(NMR) imaging. The subject method and apparatus can allow an improvedsignal to noise ratio and is particularly advantageous for applicationto vertical field NMR imaging.

There are numerous examples of surface and volume coils described in theliterature and available as commercial products. Several of theseexamples utilize multiple coils for an increased signal to noise ratioover a given field of view. In most cases, multiple coils examples havebeen applied to coil systems for use in a horizontal magnetic fields.Furthermore, the multiple coils are typically positioned to have no, orminimal, interaction with neighboring coils.

In order to image a large field of view, a first coil can be placed atone position, and one or more additional coils can be placed next to thefirst coil. If the coils interact with each other, it is preferable toswitch the coils on and off such that only one coil is on at a time.Such a coil system can be referred to as a switchable coil. If the coilsare positioned relative to one another such that they have minimal, orno, interaction, the coils can be switched on simultaneously allowingthe entire field of view to be imaged at once. Such a coil system can bereferred to as a phased array coil. The resultant image can have thesignal to noise ratio of a small coil and the field of view of a largecoil.

Advances in phased array coils have allowed linear coils to bepositioned next to other linear coils, quadrature coils to be positionednext to other quadrature coils, and volume coils to be positioned nextto other volume coils. In most cases, these coils have minimal mutualinductance and/or utilize some cancellation networks to reduce coupling.This concept can be applied to cover a larger area with several smallercoils.

U.S. Pat. No. 4,825,162 (Roemer et al.) discloses the use of multiplenoninteracting coils to acquire an NMR image. In U.S. Pat. No.4,825,162, the disclosed coils utilize simple linear designs that areintended to be used in a horizontal magnet system. These designs have noor minimal mutual inductance between the various coils, due to therelative position and/or the use of additional decoupling circuitry.However, while the goal of the Roemer device is to extend the field ofview while preserving the signal to noise, the device finds limitedapplication because two or more linear coils cannot be positioned to seethe same field of view while preserving the signal to noise ratio. Also,since the minimization of mutual inductance is the first criteria forisolation, secondary methods are then used to improve the isolationfurther.

U.S. Pat. No. 5,394,087 (Molyneaux) discloses the use of quadraturecoils positioned to minimize interaction between coils in order toachieve a higher signal to noise ratio than linear coils, whileachieving a larger field of view compared to a single quadrature coil inhorizontal field configurations. In U.S. Pat. No. 5,951,474 (Matsunagaet al.), the use of similar geometries to those disclosed in U.S. Pat.No. 5,951,474 is described for vertical field configurations. U.S. Pat.No. 5,258,717 discloses volume coils overlapped in the direction of themain field to extend the field of view, while preserving the signal tonoise of a single volume coil for horizontal configurations. A majordisadvantage of the configuration disclosed in the Molyneaux patent, theMatsunaga, et al. patent, and the Misic, et al. patent is the inabilityto use two or more linear coils positioned to see the same field of viewwhile preserving the signal to noise where only one quadrature coil seesthe same field of view. Also, the configurations disclosed in theMolyneaux and Misic et al. patents are designed to work primarily withhorizontal fields. Although the Matsunaga et al. device is intended foruse in vertical fields, considerable coupling can occur between adjacentquadrature coils, negatively impacting the signal to noise ratio.

U.S. Pat. No. 4,766,383 (Fox et al.) and U.S. Pat. No. 5,185,577(Minemura) describe configurations utilizing crossed ellipse coils, suchthat two ellipsical coils are positioned to be orthogonal to one anotherfor quadrature detection. The output is then sent to a quadraturecombiner. A major disadvantage of the configurations disclosed in theFox et al. and Minemura patents is that the crossed ellipse coils areused as quadrature coils, not array coils, and can substantiallyincrease the signal to noise as compared to a solenoid.

U.S. Pat. No. 5,351,688 (Jones) describes the use of solenoids in aquadrature fashion, where one solenoid is used for the first directionof quadrature detection and a pair of solenoids are hooked together tomake a Helmholtz pair in the second direction. Again, the output is sentto a quadrature combine. The major disadvantage of the configuration inthe Jones patent is that the solenoid is used with a set of solenoidsthat are configured as a Helmholtz pair and then fed into a quadraturecombine. This results in no increased field of view and no significantincrease in signal to noise due to the addition of the Helmholtz coils,as the center of the Helmholtz coils is far away from the center of thesingle solenoid and the field sensitivity drops as the square of thedistance away from the center of the loop.

U.S. Pat. No. 4,725,779 (Hyde, et al.) and U.S. Pat. No. 4,721,913(Hyde, et al.) disclose the use of a single or multiple loop gapresonators forming linear coils. The loop gap resonators consist ofopposite rotating current coils and planar pair coils. A significantdisadvantage of the apparatus disclosed in U.S. Pat. Nos. 4,725,779 and4,721,913 is the use of a single linear coil (either opposite rotatingor planar pair) with reduced sensitivity over a solenoid coil. U.S. Pat.No. 4,866,387 (Hyde, et al.) discloses an opposite rotating current loopgap resonator and a planar pair which are combined to form a quadraturecoil. U.S. Pat. No. 4,866,387 also discloses a plurality of planar pairand opposite rotating coils which are positioned adjacent to one anotherto form a network of coils. A drawback with respect to the configurationdisclosed in U.S. Pat. No. 4,866,387 is the use of orthogonality for theisolation of overlapping and adjacent structures.

BRIEF SUMMARY OF THE INVENTION

The subject invention pertains to a method and apparatus for NuclearMagnetic Resonance (NMR) imaging. The subject method and apparatus areparticularly advantageous with respect to the use of RF coils forreceiving signals in NMR scanners. In a specific embodiment, the subjectmethod and apparatus can utilize multiple coils to, for example, improvethe signal to noise, increase the coverage area, and/or reduce theacquisition time. The use of multiple smaller surface or volume coils toreceive NMR signals from the sample can increase the signal to noiseratio compared to a larger coil that has the same field of view andcoverage area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the subject invention utilizingthree single loops.

FIG. 2 illustrates an embodiment of the subject invention utilizingthree series loops.

FIG. 3 illustrates an embodiment of the subject invention utilizingthree parallel loops.

FIG. 4 illustrates an embodiment of the subject invention utilizingthree loops and a Helmholtz pair of large loops.

FIG. 5. illustrates an embodiment of the subject invention utilizingthree loops, a Helmholtz pair top loops, and a Helmholtz pair bottomloops.

FIG. 6 illustrates an embodiment of the subject invention utilizingthree loops and two Helmholtz pairs side-by-side loops.

FIG. 7 illustrates an embodiment of the subject invention utilizingcrossed ellipse loops and an independent loop.

FIG. 8 illustrates an embodiment of the subject invention utilizingcrossed ellipse loops and a pair of opposite current loops.

FIG. 9A illustrates an embodiment of the subject invention utilizingcrossed ellipse loops and three loops.

FIG. 9B illustrates an embodiment of the subject invention utilizingcrossed ellipse loops, a center loop, and an Alderman-Grant loop.

FIG. 10 illustrates an embodiment of a switching network for use with athree loop embodiment of the subject invention.

FIG. 11 illustrates another embodiment of a switching network for usewith a three loop embodiment of the subject invention.

FIG. 12 illustrates a capacitive network which can be used to cancelmutual inductance between a single loop and crossed ellipse loops.

FIG. 13 illustrates an embodiment of the subject invention utilizingfive loops.

FIG. 14A illustrates example even symmetry field patterns down thecentral axis produced by various drive currents for the loopconfiguration shown in FIG. 13.

FIG. 14B illustrates example odd symmetry field patterns down thecentral axis produced by various drive currents for the loopconfiguration shown in FIG. 13.

FIG. 15A illustrates an embodiment of the subject invention utilizingcrossed ellipse loops.

FIG. 15B illustrates a co-rotating current crossed ellipseconfiguration, in accordance with the subject invention.

FIG. 15C illustrate an opposite current crossed ellipse configuration,in accordance with the subject invention.

FIG. 16A illustrates a two-ring coil configuration such that thediameters, position, and relative currents in the rings are adjustedsuch that the EMF induced in ring #1 by B1 _(Ext.)(x₁) is equal andopposite to the EMF induced in ring #2 by B1 _(Ext.)(x₂), where B1_(Ext) is produced by a current in an external coil.

FIG. 16B shows a plot resulting from a Biot-Savart static calculation ofthe x-component B₁-field produced by the two-ring coil configurationshown in FIG. 16A in the X-Y plane (Z=0 cm), where the dashed linesrepresent three different external circular surface coils of differentdiameters and positions which have zero net flux (arrows indicatedirection of B_(x)-field) and are therefore isolated from the two-ringcoil configuration.

DETAILED DESCRIPTION OF THE INVENTION

The subject invention relates to a method and apparatus for NuclearMagnetic Resonance (NMR) imaging. The subject method and apparatus cantake advantage of the properties of a system utilizing a solenoidlocated at or near essentially zero magnetic field planes of othersolenoid configurations, and are particularly advantageous with respectto the use of RF coils for receiving signals in NMR scanners. The use ofmultiple smaller surface or volume coils to receive NMR signals from thesample can increase the signal to noise ratio compared to a larger coilthat has the same field of view and coverage area. Accordingly, thesubject method and apparatus can utilize multiple coils to, for example,improve the signal to noise, increase the coverage area, and/or reducethe acquisition time.

In the subject application, for ease of description, the coilconfigurations are often described in relation to the fields they wouldcreate if driven as generating coils. It is understood that thisdescription, due to reciprocity between generating fields and receivingfields with a coil configuration, can also be descriptive of coilconfigurations for receiving magnetic fields. It is understood that acoil configuration associated with a magnetic field can be used forgenerating such magnetic field, detecting such magnetic field, orgenerating and detecting such magnetic field. In order to generate andreceive, a coil configuration can, for example, be driven by a means forutilizing the coils for generating magnetic fields and then, afterdiscontinuing the generation of the fields, a means for utilizing thecoils for detecting can be employed for detecting magnetic fieldsassociated with the coils. Accordingly, a zero-flux plane with respectto coil configuration designed for producing magnetic fields can be aplane in which any magnetic field would not be detected by the same coilconfiguration when used for receiving, or detecting, magnetic fields.

In a specific embodiment of the subject invention, three coils arepositioned in a configuration such that two outer coils are coaxial, liein parallel planes, and are driven by currents of opposite direction. Inthis three coil configuration, the center coil lies in a plane parallelto the planes of the outer two coils and is located near an essentiallyzero magnetic flux plane produced by the outer coils. If the outer coilshave essentially identical shape and size, and the currents in the outercoils are approximately equal, such zero magnetic flux plane will beapproximately at the midpoint between the planes of the two outer coils.The position of this zero-flux plane with respect to the relativedistance to each outer coil can be adjusted by changing the ratio of themagnitudes of the currents in the outer two coils. The shape of thezero-flux plane may change to some more complicated zero-flux contour aswell.

Although the parallel, coaxial orientation of the outer coils ispreferred, other configurations are also possible. For example, the twoouter coils can be non-coaxial, but still lie in parallel planes. Thisembodiment can be useful for imaging, for example, a person's head whenthe head leans forward. In this case, the zero-flux contour may not be aplane, but a more complicated geometry. In another embodiment, the twoouter coils are non-coaxial and do not lie in parallel planes. Thisembodiment can be useful for imaging, for example, a person's shoulder,where the non-parallel coils can better conformn to the shape of theshoulder. In addition, although loops of the approximate same size arepreferred, different sized loops can also be used together.

FIG. 1 shows a preferred embodiment employing three parallel loops, 1B,2B, and 3B, of the same size and shape. In this embodiment, loops 1B and3B are approximately equidistant from the central loop 2B and designedsuch that approximately equal currents can flow through them in oppositedirections. The central loop, 2B, can be considered a first channel andthe other two loops 1B and 3B can be considered a second channel.Although the subject invention can be utilized even when significantcoupling between the two channels exists, preferably the coils aredesigned such that the coupling between the two channels is extremelylow. Isolation circuitry can optionally be incorporated to reducecoupling. Advantageously, this design employing a single loop as a firstchannel and a pair of loops as a second channel can result in the pairof loops producing approximately zero magnetic flux in the plane of thesingle loop.

Connections can be made between loops 1B and 3B such that the loops havecurrents flowing in opposite directions. These connections can be, forexample, series or parallel connections. Although, the solenoids shownin FIG. 1 are single turns, or loops, the solenoids can be multiturnsolenoids and can be wound as series loops or parallel loops. FIG. 2shows an embodiment utilizing series loops and FIG. 3 shows anembodiment utilizing parallel loops. Referring to FIGS. 2 and 3, theconnections between 4A and 6A, as well as 7A and 9A, may be series orparallel as long as the currents are flowing in opposite directions inthe two loops.

The method and apparatus of the subject invention can also incorporateat least one Helmholtz pair which generates a magnetic field orthogonalto the fields produced by the solenoids of FIG. 1. FIGS. 4, 5, and 6illustrate a few examples where Helmholtz pairs are added to theembodiment of FIG. 1. Although FIGS. 4, 5 and 6 illustrate theincorporation of these Helmholtz coils with a three coil embodiment,these Helmholtz coils can be incorporated with other embodiments of thesubject invention as well, for example embodiment having additionalcoils, additional channels, different coil orientations, and/ordifferent size coils. Once again, even though the loops in FIGS. 4, 5,and 6 are shown as single turns, or loops, the loops that make up aHelmholtz pair can also be multiturn solenoids and can be wound asseries loops or parallel loops. Also, the connections between 26 and 27,31 and 32, 33 and 34, 37 and 40, and 38 and 39 can be series or paralleland allow approximately equal currents to flow in the two loops of thepair.

FIG. 4 shows an embodiment incorporating a “large loops” Helmholtz coilpair. Although the preferred embodiment of the large loops Helmholtzpair is shown, where the large static magnetic field used during NMR isoriented from bottom to top of the Figure, this pair can be rotatedabout the central axis of the cylinder formed by loops 23, 24 and 25.Also an additional large loop Helmholtz coil pair can be utilized ifdesired. For example, an additional large loop Helmholtz coil pair canbe added to the top and bottom of the embodiment shown in FIG. 4 suchthat essentially the entire cylinder formed by loops 23, 24, and 25 issurrounded by the two large loop Helmholtz coil pairs.

FIG. 5 shows an embodiment which incorporates top/bottom loops. In thisembodiment coil 31 and coil 32 form a top coil pair and coil 33 and coil34 form a bottom coil pair. Preferably, the coil pairs overlap such thatmutual inductance between coil 32 and coil 33 and between coil 31 andcoil 34 is low. Most preferably, the amount of overlap can be selectedso as to achieve approximately zero mutual inductance. Additional coilpairs can be added and/or the coil pair(s) can be rotated with respectto the central axis of the cylinder formed by loops 28, 29, and 30.

FIG. 6 shows an embodiment of the subject invention incorporating sideby side loops. Loops 37 and 40 form one loop pair and loops 38 and 39form another. Preferably the amount of overlap of side by side looppairs is chosen so that the mutual inductance of the loops is low, and,more preferably, the amount of overlap is chosen so that the mutualinductance is approximately zero. Additional loops can be added to oneor more side by side pairs and/or additional side by side pairs can beadded. Again, the side by side pairs can be rotated with respect to thecentral axis of the cylinder formed by loops 34, 35, and 36.

Another embodiment of the subject invention incorporates a crossedellipse geometry with the three solenoid model shown in FIG. 1, anexample of which is shown in FIG. 9A. The single solenoid can be removedif desired and the opposite current solenoids can be utilized with thecrossed ellipse, as shown in FIG. 8. Alternatively, the opposite currentsolenoids can be removed and the independent solenoid can be utilizedwith the crossed ellipse, as shown in FIG. 7. For ease of illustration,the crossed ellipses have been drawn such that their “intersection” isnot at the top and bottom of the cylinder. Preferably, the intersectionof the crossed ellipses (note the ellipses do not necessarily come intoelectrical contact at this intersection) is at the top and bottom ofFIGS. 7, 8 and 9A, where the large external static magnetic field isoriented from the bottom to the top of the Figures. Most preferably, theintersection of the crossed ellipses is at the single loop, if present.

With respect to the embodiments shown in FIG. 7 and FIG. 9A, FIG. 12illustrates a specific embodiment of a capacitive network which can beused to minimize or cancel mutual inductance between the single solenoidand the crossed ellipse. Referring to FIGS. 12, 10A and 10A′ representthe contacts for ellipse 10B and are analogous to contacts 14A and 14A′for ellipse 14B of FIG. 9A; 12A and 12A′ represent the contacts for loop12B and are analogous to contacts 17A and 17A′ for loop 17B of FIGS. 9A;and 11A and 11A′ represent the contacts for ellipse 11B and areanalogous to contacts 15A and 15A′ for ellipse 15B of FIGS. 9A. C1, C2,C3, and C4 are four capacitive elements of the capacitive network shownin FIG. 12.

In a specific embodiment, the crossed ellipse/opposite rotatingconfiguration shown in FIG. 8 can be simplified by the superposition ofthe opposite rotating mode on the crossed ellipse conductors as shown inFIG. 15. In a specific embodiment, the two loops that form the oppositerotating mode in FIG. 8 can be removed, and the opposite rotating modecan be superimposed onto the crossed ellipse. In this embodiment, thecrossed ellipse configuration can support two linear orthogonal modes,one for each loop, and a third mode which represents the oppositerotating mode. Alternatively, the crossed ellipse configuration cansupport two linear orthogonal modes, each a superposition of individualmodes associated with each of the two coils. The opposite rotating modecan be isolated from the two linear orthogonal modes due to zero mutualinductance. Referring to FIGS. 8, 15B, and 15C, loop 15B can produce afirst linear mode 100 of the crossed ellipse, and loop 14B can produce asecond linear mode 101 of the crossed ellipse which is orthogonal to thefirst mode. The opposite rotating mode 103 of the crossed ellipse isshown in FIG. 15C where the crossed ellipses have been broken apart in amanner to emphasize the currents for producing the opposite rotatingmode 103. Reference points 98 and 99 shown on FIGS. 15A, 15B, and 15Cillustrate points at which the current can change directions to producetwo linear orthogonal modes or the opposite rotating mode. Coupling tothe structure can be achieved through capacitive or inductive methods.If desired, the opposite rotating mode can be produced on a secondcrossed ellipse coil pair aligned with the first coil pair.

Referring to the embodiment of the subject invention shown in FIG. 9A,coil 17B can act as a solenoid around the center of the region ofinterest. Coils 14B and 15B can form crossed ellipse coils, and coils16B and 18B can form an opposite rotating coil centered on coil 17B. Theopposite rotating coil can be isolated via symmetry from coils 17B, 14B,and 15B. 14A, 15A, 16A, 17A, and 18A show the contacts for the variouscoils. Coils 14B and 15B can be isolated from one another by, forexample, having their axes perpendicular to each other. In thisarrangement, Coil 17B can have strong mutual inductance with both coils14B and 15B. This inductance can be isolated by using one or more ofvarious techniques known to those skilled in the art. The oppositerotating coil can have a zero flux in the center and improve thehomogeneneity of the coverage by producing fields away from the center.Advantageously, the embodiment of FIG. 9A can produce excellenthomogeneity down the axis of the cylinder.

In another embodiment of the subject invention, as shown in FIG. 9B,coil pair 16B and 18B can be modified so as to produce an Alderman-Grant(Alderman, D. W. and Grant, D. M., Jo. Magnetic Resonance 36:447 [1979])type of coil, such that coil 17B is isolated from the Alderman-Grantcoil due to the fields of the Alderman-Grant being perpendicular to thefields of coil 17B. Such an Alderman-Grant coil can be achieved byadding a pair of conductors 30 and 31 to connect coils 16B and 18B suchthat conductors 30 and 31 carry the same magnitude current in oppositedirections. The currents flowing in conductors 30 and 31 are split whenthe currents enter a coil, with one-half the magnitude of the currentflowing in each half of the coil. For example, current flowing fromconductor 30 flows one-half in each half of coil 18B to conductor 31,and current flowing from conductor 31 flows one-half in each half ofcoil 16B to conductor 30. 14A, 15A, and 17A show the contacts for thevarious coils, and 30A shows the contacts for the Alderman-Grant coil.In this embodiment, coils 17B and the Alderman-Grant coil are isolateddue to their perpendicular fields and coils 14B and 15B are isolatedfrom one another by, for example, having their axes perpendicular toeach other. Coil 17B shares inductance and sample resistance with coils14B and 15B, and the Alderman-Grant coil shares inductance andresistance with coil 14B and 15B.

With respect to the embodiment shown in FIG. 9A, shared resistancebetween coils 17B and 14B and between coils 17B and 15B can limit theisolation. Preamplifier decoupling can increase the isolation toacceptable values. Also, due to the shared resistance, the amount ofnoise correlation in these channels is a consideration which should betaken into account.

FIGS. 10 and 11 illustrate switching networks which can be utilized withrespect to the three solenoid embodiment, for implementing a method toallow the opposite rotation of the loop currents in either a series orparallel fashion. FIG. 10 shows a switching network for allowing theouter two coils to have currents which either rotate in the samedirection or in opposite directions. Referring to FIGS. 10, 1A′, 2A′,and 3A′ connect to the top contacts of loops 1B, 2B, and 3B of FIG. 1,while 1A, 2A, and 3A connect to the bottom contacts. By closing switches50 and 53, loops 1B and 3B can be driven in the same rotation direction.By closing switches 51 and 52 and opening switches 50 and 53, loops 1Band 3B can be driven in opposite rotation direction. Analogously, 1A′,2A′, and 3A. of FIG. 11 can connect to the top contacts of loops 4B, 5B,and 6B (4A′, 5A′, and 6A′, respectively) of FIG. 2, while 1A, 2A, and 3Aof FIG. 11 connect to the bottom contacts (4A, 5A, and 6A,respectively). Analogously, 1A′, 2A′, and 3A′ of FIG. 11 can connect tothe top contacts of loops 7B, 8B, and 9B (7A′, 8A′, and 9A′,respectively) of FIG. 3, while 1A, 2A, and 3A, of FIG. 11 connect to thebottom contacts (7A, 8A, and 9A, respectively).

FIG. 11 illustrates a more generalized switching network which can allowall three coils to have currents rotating in the same direction, orallow the center coil to operate independently, while the two outer coilcurrents either rotate in the same direction or rotate in oppositedirections. Referring to FIGS. 11, 1A′, 2A′, and 3A′ connect to the topcontacts of loops 1B, 2B, and 3B of FIG. 1, while 1A, 2A, and 3A connectto the bottom contacts. The network is driven at lead pairs 54 and 55.If switches 50 and 53 are closed, loops 1B and 3B are driven in the samerotation direction with the center coil independent. By closing switch56, loop 2B can be driven at lead pairs 55. By closing switches 60, 57,56, 59, and 58, with switches 50 and 53 open, all three loops 1B, 2B and3B can be driven in the same direction rotation so as to function as athree turn solenoid. By closing switches 51 and 52 and opening switches50 and 53, loops 1B and 3B can be driven in opposite rotation direction.The network can also allow the three loops to operate as one loop formultiturn solenoid operation. The switching networks shown in FIGS. 10and 11 can be modified to incorporate additional loops and/or channelsas appropriate.

The method and apparatus of the subject invention can also be extendedto more than two channels. With respect to the three solenoid exampleshown in FIG. 1, the single loop coil can function as a first channelhaving even symmetry, while the two outer loop coils can function as asecond channel having odd symmetry. In alternative embodiments, thesingle loop coil can be replaced by one of a variety of loopconfigurations having multiple loops which maintain even symmetry and,accordingly, preserve the isolation of the channels. For example, aconfiguration having multiple loops can be utilized such that loop pairscarrying equal currents are placed equidistant from the plane definingeven and odd symmetry. For non-equal currents, the distance each loop isfrom the center plane can be adjusted such that the even symmetry ismaintained.

Furthermore, additional loop configurations with even symmetry can beadded to create additional channels. These even modes can have an evennumber (including zero) nodes, or zero flux contours. In a specificembodiment, a third channel, having even symmetry and two nodes can beincorporated. Such a third channel can be configured such that the twonodes are positioned in the magnetic field such that the net currentinduced in the first channel, of even symmetry, is approximately zerowhen the third channel is driven. Advantageously, both even symmetrychannels can remain isolated from the odd symmetry channel. Furtherembodiments of the subject invention can add additional even symmetrychannels having a different number of nodes in an analogous manner whereeach even symmetry channel having a different number of nodes can beorthogonal to each of the other even symmetry channels.

Likewise, additional loop configurations with odd symmetry can be addedto create additional channels. These odd modes can have an odd number ofnodes, or zero flux contours. Advantageously, each odd modes having adifferent number of nodes can be orthogonal to each of the other oddmodes and can be orthogonal to all even modes.

Referring to FIG. 13, five loops oriented coaxially to one another, withbilateral symmetry around the center loop A, B, C, D, and E, are shown.Bilateral symmetry means any loop on one side of the center loop is thesame distance to the center loop as a similar loop on the other side ofthe center loop. These five coaxial loops can be used to produce fivecurrent patterns that have negligible mutual inductance between eachpair of patterns in a region of interest. Three of these currentpatterns have even symmetry, while two have odd symmetry around thecenter loop. The odd symmetry patterns are required to have zero currentin the center loop, since odd symmetry of currents means that a loop onone side of center has opposite rotating current to the similar loop onthe other side of center. Even symmetry of current requires a loop onone side of center to have equal current in the same direction to asimilar loop on the other side of center. All even symmetry patternswill inherently have zero mutual inductance with all odd symmetrypatterns. FIGS. 14A and 14B show example field patterns down the centralaxis of the loops which can produced by certain current combinations forthe loop configuration shown in FIG. 13. The field patterns shown inFIGS. 14A and 14B illustrate how negligible mutual inductance can beproduced between various field patterns.

With respect to embodiments described having opposite-rotating currentsin a coaxial coil pair where the coils lie in a plane and areapproximately the same size, the zero-flux contour typically liesbetween the two coils. In addition, two additional zero-flux contourscan be considered to lie at infinity in either direction. However, ifthe parameters of the coil pair configuration are adjusted, one of thezero-flux contours of infinity can be brought toward the coil pair suchthat it remains outside the coil pair but is close enough to the coilpair to be useful with respect to the subject invention. In this case,another coil configuration can be placed at such zero-flux contouroutside the coil pair.

Various embodiments illustrated in the Figures utilize coils residing ina plane. The subject apparatus and method are also applicable to coilsnot residing in a plane, where the subject invention can take advantageof essentially zero-flux contours which do not necessarily lie in aplane. By locating a coil in the zero-flux contours produced by othercoil configurations, an independent channel can be achieved.

Referring to FIG. 16A, an external coil is shown at X₀ which can producea non-uniform B-field at the position of a coil pair configuration, onecoil at X₁ and a second coil at X₂. The coil at X₁ can have a radius R₁and current I₁ while the coil at X₂ can have a radius R₂ and a currentI₂. The coil pair can be adjusted such that the net electromagneticforce (EMF) caused by flux through the coils is zero, and, thus, netcurrent is zero. In this way, the coil pair can be considered isolatedfrom the external coil. By the principle of reciprocity, with such anarrangement, any B-field produced by the coil pair should produce a netB-flux through the external coil of zero as well. The coil pair canproduce a B-field with a component that has a zero crossing in a planethrough the external coil, as shown in FIG. 16B. In this way, bothpositive and negative contributions to the total B-flux exist whichcancel upon integration over said plane. This is what is referred to asa zero-flux contour. One skilled in the art will recognize that this isequivalent to saying that the said coil and the external coil pair havezero mutual inductance.

As discussed, the subject invention can incorporate a plurality of coilssuch that a plurality of modes can be supported. For example, N loopswith bilateral symmetry can support N-modes, corresponding to currentpatterns that all have zero net mutual inductive coupling to one anotherin a region of interest. The current patterns associated with thesemodes can be computed in a straight forward manner. As an example, thegeneral case of N=5 loops, the matrix representing the inductivecoupling from one mode to another can be written as:

L₁ M₁₂ M₁₃ M₁₄ M₁₅ M₂₁ L₂ M₂₃ M₂₄ M₂₅ M₃₁ M₃₂ L₃ M₃₄ M₃₅ M₄₁ M₄₂ M₄₃ L₄M₄₅ M₅₁ M₅₂ M₅₃ M₅₄ L₅

In general, M_(ij)=M_(ji). For the special case of equivalent loops, theinductance (L) is identical for all loops. Additionally, M₁₂=M₄₅,M₂₃=M₃₄, M₁₄=M₂₅ and M₁₃=M₃₅ for a bilaterally symmetric arrangement ofthe five loops as shown, for example, in FIG. 13. Therefore theinductive coupling matrix can be written:

L M₁₂ M₁₃ M₁₄ M₁₅ M₁₂ L M₂₃ M₂₄ M₁₄ M₁₃ M₂₃ L M₂₃ M₁₃ M₁₄ M₂₄ M₂₃ L M₁₂M₁₅ M₁₄ M₁₃ M₁₂ L

The eigenvectors of this matrix can be computed using ordinary means.The eigenvectors represent vectors of the form $\begin{pmatrix}i_{1} \\i_{2} \\i_{3} \\i_{4} \\i_{5}\end{pmatrix}.$

The five sets of current patterns corresponding to the eigenvectors areorthogonal and therefore have zero effective mutual inductance. It canbe shown that any symmetrical arrangement of N equivalent loops has acoupling matrix that has non-degenerate, orthogonal eigenvectors andthus N current patterns that are isolated from one another.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

What is claimed is:
 1. A magnetic resonance imaging system coilconfiguration, comprising: a radio frequency pair of coils in anopposite rotation orientation associated with a first magnetic field ina region of interest, wherein the region of interest is essentiallywithin a cylinder created by the pair of coils; a radio frequency singlecoil associated with a second magnetic field in the region of interest,wherein the first magnetic field and the second magnetic field aresubstantially parallel in the region of interest, wherein the singlecoil is positioned at an essentially zero-flux contour with respect tothe first magnetic field.
 2. The configuration according to claim 1,further comprising: a means for utilizing the pair of coils fordetecting the first magnetic field; and a means for utilizing the singlecoil for detecting the second magnetic field.
 3. The configurationaccording to claim 1, further comprising: a means for utilizing the pairof coils for generating the first magnetic field; and a means forutilizing the single coil for generating the second magnetic field. 4.The configuration according to claim 1, wherein the zero-flux contour islocated outside the pair of coils with respect to the length of thecylinder created by the pair of coils.
 5. The configuration according toclam 1, wherein said pairs of coils are connected together by a pair ofelectrical conductors to form an Alderman-Grant coil pair.
 6. Theconfiguration according to claim 1, wherein each of said pair of coilsand said single coil lie in planes parallel to each other, and whereinsaid essentially zero-flux contour is an essentially zero-flux plane. 7.The configuration according to claim 6, wherein the pair of coils andthe single coil are co-axial.
 8. The configuration according to claim 2,wherein the single coil is a first channel and the pair of coils is asecond channel such that coupling between the first channel and secondchannel is low.
 9. The configuration according to claim 8, whereincoupling between the first channel and second channel is approximatelyzero.
 10. The configuration according to claim 1, wherein the zero-fluxcontour is located between the pair of coils.
 11. The configurationaccording to claim 10, wherein a second zero-flux contour with respectto the fit magnetic field is located outside the pair of coils, furthercomprising a second single coil for generating a third magnetic field inthe region of interest, wherein the second single coil is positioned atthe second zero-flux contour with respect to the first magnetic field.12. The configuration according to claim 10, wherein the single coil ispositioned approximately equidistance from each of the pair of coils.13. The configuration according to claim 10, wherein the single coil ispositioned closer to one of the coils of the pair of coils than to theother.
 14. The configuration according to claim 2, further comprising: ameans for utilizing the pair of coils for generating the first magneticfield; and a means for utilizing the single coil for generating thesecond magnetic field.
 15. The configuration according to claim 1,further comprising: at least one Helmholtz coil pair associated with athird magnetic field essentially orthogonal to the frost and secondmagnetic fields in the region of interest.
 16. The configurationaccording to claim 15, further comprising a means for utilizing said atleast one Helmholtz coil pair for generating the third magnetic field.17. The configuration according to claim 15, wherein said Helmholtz coilpair is of a configuration selected from the group consisting of: largeloops, top/bottom loops, side by side loops, and a combination thereof.18. The configuration according to claim 2, wherein said coils of saidpair of coils and said single coil are selected from the groupconsisting of: a single turn loop, a multiturn solenoid wound as seriesloops, and a multiturn solenoid wound as parallel loops.
 19. Theconfiguration according to claim 1, further comprising: a switchingmeans for allowing the pair of coils and the single coil to operate inand switch between two or more of the modes in the group consisting of:(i) the coils of the pair of coils and the single coil having currentsrotating in the same direction; (ii) the coils of the pair of coilshaving currents rotating in the same direction, with the single coiloperating independently; (iii) the coils of the pair of coils havingcurrents rotating in opposite directions, with the single coil operatingindependently; and (iv) the coils of the pair of coils having currentsrotating in the same direction and the single coil having a currentrotating in an opposite direction with respect to the currents of thepair of coils.
 20. The configuration according to claim 1, furthercomprising: at least one additional pair of coils, wherein said pair ofcoils in an opposite orientation has odd symmetry with respect to aplane, wherein each of said at least one additional pair of coils isassociated with a corresponding at least one additional magnetic field,wherein each of said at least one additional pair of coils has evensymmetry with respect to the plane and is associated with one of said atleast one additional magnetic field such that said single coil is afirst channel, said pair of coils in an opposite orientation is a secondchannel, and each of said at least one additional pair of coils is anadditional channel which is orthogonal to the first channel, secondchannel, and each of the other additional channels.
 21. Theconfiguration according to claim 20, wherein each of said pair of coils,said single coil, and each of said at least additional pairs of coilslie in planes parallel to each other.
 22. The configuration according toclaim 21, wherein each of said pair of coils, said single coil, and eachof said at least additional pairs of coils are coaxial.
 23. Theconfiguration according to claim 1, further comprising: at least oneadditional pair of coils, wherein said pair of coils in an oppositeorientation has odd symmetry with respect to a plane, wherein each ofsaid at least one additional pair of coils is associated with acorresponding at least one additional magnetic field, wherein each ofsaid at least one additional pair of coils has odds symmetry withrespect to the plane and is associated with one of said at least oneadditional magnetic field such that said single coil is a first channel,said pair of coils in an opposite orientation is a second channel, andeach of said at least one additional pair of coils is an additionalchannel which is orthogonal to the first channel, second channel, andeach of the other additional channels.
 24. The configuration accordingto claim 23, wherein each of said pair of coils, said single coil, andeach of said at least additional pairs of coils lie in planes parallelto each other.
 25. The configuration according to claim 24, wherein eachof said pair of coils, said single coil, and each of said at leastadditional pairs of coils are coaxial.
 26. The configuration accordingto claim 1, wherein the pair of coils and single coil are positionedwith respect to an external static magnetic field such that thedirection of the external static magnetic field is perpendicular to theaxis of the cylinder created by the pair of coils.
 27. A method ofdetecting magnetic fields in a magnetic resonance imaging system,comprising the following steps: detecting a first magnetic field in aregion of interest utilizing a radio frequency pair of coils in anopposite rotation orientation associated with the first magnetic fieldin the region of interest, wherein the first magnetic field and thesecond magnetic field are substantially parallel in the region ofinterest, wherein the region of interest is essentially within acylinder created by the pair of coils; and detecting a second magneticfield in the region of interest utilizing a radio frequency single coilassociated with a second magnetic field in the region of interest,wherein the single coil is positioned at an essentially zero-fluxcontour with respect to the first magnetic field.
 28. A magneticresonance imaging system RF coil configuration, comprising: at leastfive RF coils with bilateral symmetry, wherein the at least five RFcoils are coaxial, wherein said at least five RF coils are associatedwith a plurality of modes such that the number of modes is less than orequal to the number of RF coils, wherein said plurality of modescorrespond with a plurality of current patterns, each of said pluralityof current patterns having zero net mutual inductive coupling to each ofthe other of said plurality of current patterns in a region of interest.29. The configuration according to claim 28, further comprising: a meansfor utilizing the plurality of RF coils for detecting magnetic fieldsassociated with the plurality of current patterns.
 30. The configurationaccording to claim 28, further comprising: a means for utilizing theplurality of RF coils for generating magnetic fields associates with theplurality of current patterns.
 31. A method of detecting magnetic fieldsin a magnetic resonance imaging system, comprising the flooring steps:positioning at least five RF coils coaxially with respect to a region ofinterest such that the at least five RF coils support a plurality ofmodes corresponding to a plurality of current patterns; and detectingthe plurality of modes associated with the at least five RF coils,wherein the number of RF coils is greater than or equal to the number ofmodes, and wherein each of the plurality of current patterns has zeronet mutual inductive coupling to each of the other of the plurality ofcurrent pattern in a region of interest.
 32. A method of detectingmagnetic fields in a magnetic resonance imaging system, comprising:positioning a radio frequency pair of coils in an opposite rotationorientation, wherein the pair of coils are associated with a firstmagnetic field in a region of interest, wherein the region of interestis essentially within a cylinder created by the pair of coils;positioning a radio frequency single coil at an essentially zero-fluxcontour with respect to the first magnetic field, wherein the singlecoil is associated with a second magnetic field in the region ofinterest; detecting the first magnetic field with the pair of coils; anddetecting the second magnetic field with the single coil.
 33. The methodaccording to claim 32, wherein the zero-flux contour is located outsidethe pair of coils, wherein the region of interest extends outside thepair of coils.
 34. The method according to claim 32, further comprising:positioning an object to be imaged in the region of interest.
 35. Themethod according to claim 32, wherein positioning an object to be imagedin the region of interest comprises positioning an object to be imagedsuch that at least a portion of the object is in the essentiallyzero-flux contour with respect to the first magnetic field.
 36. Themethod according to claim 35, wherein each of said pair of coils andsaid single coil lie in planes parallel to each other, and wherein saidessentially zero-flux contour is an essentially zero-flux plane, whereinpositioning an object to be imaged in the region of interest comprisespositioning an object to be imaged such that at least a portion of theobject is in the essentially zero-flux plane.
 37. The method accordingto claim 34, wherein positioning an object to be imaged in the region ofinterest comprises inserting at least a portion of the object to beimaged into the region of interest through one of the pair of coils. 38.The method according to claim 37, wherein positioning an object to beimaged in the region of interest further comprises inserting at least aportion of the at least a portion of the object to be imaged through thesingle coil.
 39. The method according to claim 32, further comprisinggenerating the first magnetic field with the pair of coils, andgenerating the second magnetic field with the single coil.
 40. Themethod according to claim 32, wherein the single coil is a first channeland the pair of coils is a second channel such that coupling between thefirst channel and second channel is low.
 41. The method according toclaim 40, wherein coupling between the first channel and second channelis approximately zero.
 42. The method according to claim 32, furthercomprising positioning the pair of coils with respect to an externalstatic magnetic field such that the direction of the external staticmagnetic field is perpendicular to the axis of the cylinder created bythe pair of coils.
 43. The method according to claim 42, whereinpositioning the pair of coils with respect to an external staticmagnetic field comprises positioning the pair of coils with respect to avertical external static magnetic field.
 44. A method of coilconfiguration for a magnetic resonance imaging system, comprising:positioning at least five RF coils with bilateral symmetry, wherein theat least five RF coils are coaxial, wherein said at least five RF coilsare associated with a plurality of modes such that the number of modesis less than or equal to the number of RF coils, wherein said pluralityof modes correspond with a plurality of current patterns, each of saidplurality of current patterns having zero net mutual inductive couplingto each of the other of said plurality of current patterns in a regionof interest; and detecting magnetic fields associated with the pluralityof modes with the at least five RF coils.
 45. The method according toclaim 37, further comprising: positioning at least one additional pairof coils, wherein said pair of coils in an opposite orientation has oddsymmetry with respect to a plane, wherein each of said at least oneadditional pair of coils is associated with a corresponding at least oneadditional magnetic field; and detecting the corresponding at least oneadditional magnetic field with the corresponding at least one additionalpair of coils, wherein each of said at least one additional pair ofcoils has odd symmetry with respect to the plane and is associated withone of said at least one additional magnetic field such that said singlecoil is a first channel, said pair of coils in an opposite orientationis a second cell, and each of said at least one additional pair of coilsis an additional channel which is orthogonal to the first channel,second channel, and each of the other additional channels.
 46. Themethod according to claim 45, wherein each of said pair of coils, saidsingle coil, and each of said at least additional pairs of coils lie inplanes parallel to each other.
 47. The method according to claim 46,wherein each of said pair of coils, said single coil, and each of saidat least additional pairs of coils are coaxial.
 48. The method accordingto claim 32, wherein the zero-flux contour is located between the pairof coils.
 49. The method according to claim 33, wherein a secondzero-flux contour with respect to the first magnetic field is locatedoutside the pair of coils, further comprising a second single coil forgenerating a third magnetic field in the region of interest, wherein thesecond single coil is positioned at the second zero-flux contour withrespect to the first magnetic field.
 50. The method according to claim48, wherein the single coil is positioned approximately equidistancefrom each of the pair of coils.
 51. The method according to claim 48,wherein the single coil is positioned closer to one of the coils of thepair of coils than to the other.
 52. The method according to claim 32,further comprising: positioning at least one Helmholtz coil pair,wherein the at least one Helmholtz coil pair is associated with a thirdmagnetic field essentially orthogonal to the first and second magneticfields in the region of interest; and detecting the third magnet fieldwith the at least one Helmholtz coil pair.
 53. The method according toclaim 52, further comprising utilizing said at least one Helmholtz coilpair for generating the third magnetic field.
 54. The method accordingto claim 52, wherein said Helmholtz coil pair is of a configurationselected from the group consisting of: large loops, top/bottom loops,side by side loops, and a combination thereof.
 55. The method accordingto claim 32, wherein said pairs of coils are connected together by apair of electrical conductors to form an Alderman-Grant coil pair. 56.The method according to claim 32, further comprising: providing aswitching means for allowing the pair of coils and the single coil tooperate in and switch between two or more of the modes in the groupconsisting of: (i) the coils of the pair of coils and the single coilhaving currents rotating in the same direction; (ii) the coils of thepair of coils having currents rotating in the same direction, with thesingle coil operating independently; (iii) the coils of the pair ofcoils having current rotating in opposite directions, with the singlecoil operating independently; and (iv) the coils of the pair of coilshaving currents rotating in the same direction and the single coilhaving a current rotating in an opposite direction with respect to thecurrents of the pair of coils.
 57. The method according to claim 32,further comprising: positioning at least one additional pair of coils,wherein said pair of coils in an opposite orientation has odd symmetrywith respect to a plane, wherein each of said at least one additionalpair of coils is associated with a corresponding at least one additionalmagnetic field; and detecting the corresponding at least one additionalmagnetic field with the corresponding at least one additional pair ofcoils, wherein each of said at least one additional pair of coils haseven symmetry with respect to the plane and is associated with one ofsaid at least one additional magnetic field such that said single coilis a first channel, said pair of coils in an opposite orientation is asecond channel, and each of said at least one additional pair of coilsis an additional channel which is orthogonal to the first channel,second channel, and each of the other additional channels.
 58. Themethod according to claim 57, wherein each of said pair of coils, saidsingle coil, and each of said at least additional pairs of coils lie inplanes parallel to each other.
 59. The method according to claim 58,wherein each of said pair of coils, said single coil, and each of saidat least additional pairs of coils are coaxial.